1. Field of the Invention
The invention relates to the field of cardiac catheters and in particular to catheters used to place stents in infants and the methods by which such catheters may be used for pediatric stent placement.
2. Description of the Prior Art
Work began as early as 1969 using nonexpandable stainless steel coils in combination with angioplasty catheter techniques for the placement of stents in dog femoral arteries C. T. Dotter, "Transluminally placed coil spring endarterial tube grafts: Long term patency in canine popliteal artery," Invest. Radiol 4:329-331 (1969). Following Dotter, several different types of stent designs were devised, but each of these designs suffer from limitations which prevented their success in clinical application. These limitations included bulky configurations that made delivery to target lesions difficult, unpredictable expansion of the device, migration due to stent-vessel mismatch, abrupt thrombosis, or gradual restenosis from intimal hyperplasia.
Evolving from these early efforts are three basic designs which include a spring-loaded stent, usually made of stainless steel and constrained to a small diameter on a delivery catheter. The stent is allowed to spring open to a predetermined diameter when the constraint is removed. See Sigwart et al. "Intravascular stents to prevent occlusion and restenosis after transluminal angioplasty," N. Engl. J. Med. 316:701-706 (1987).
A second type of stent relies upon thermal expansion. Stents made of nitinol are initially configured as small diameter wires or coils and expand to predetermined shapes with the application of body heat.
A third type of stent is introduced with a balloon catheter. Stents of this type rely on plastic deformation of the metal caused by mechanical expansion when driven by a balloon. Woven stainless steel wire stents are crimped over conventional balloon catheters and then expanded after being positioned at the target site. When the balloon is inflated, the stent is embedded in the vessel wall. Following balloon deflation and removal, the stent remains in place, holding the vessel open if properly placed initially, Palmaz et al., "Expandable intraluminal graft: Preliminary study" Radiology 156:73-77 (1985); Palmaz et al. "Expandable intraluminal grafting," Radiology 160:723-726 (1986); Palmaz et al. "Expandable intraluminal vascular graft: A feasibility study," Surgery 99:199-205 (1986); Palmaz et al. "Expandable intrahepatic portacaval shunt stents: Early experience in the dog", AJR 145:821-825 (1985). See also generally, Schatz, "Introduction to intravascular stents," Interventional Cardiology Clinics Vol. 6 No. 3 pp 357-372 (1988); Palmaz, "Balloon-expandable intravascular stent," AJR 150:1263-1269 (1988); O'Laughlin, "Use of Endovascular Stents in Congenital Heart Disease," Circulation Vol. 83, 1923-39 (1991). However, each of these prior art catheters for delivering stents did so by means of a single balloon angioplasty catheter.
In a variety of congential pediatric heart defects, there is a need for augmented pulmonary blood flow due to pulmonary atresia or stenosis. These defects include Tetralogy of Fallot, tricuspid atresia and complex forms of pulmonic stenosis. It is advantageous in these applications to maintain a patent ductus arteriosus. In other words, when blood circulation through an infant's heart is blocked or substantially restricted, blood flow from the heart to the lungs is diverted through the ductus arteriosus which is open in the infant's heart only for a relatively short period of time after birth. As the ductus arteriosus closes, as a result of normal heart development, no alternative path then remains to allow for adequate pulmonary circulation in the defective heart. In the worst cases, the infant will eventually become cyanotic and suffocate.
Conventional treatment includes drug treatments by infusions of prostaglandin which inhibits the closing of the ductus arteriosus. This treatment has a disadvantage that the infant must be kept on constant intravenous infusions of the drug until it has become large enough to survive cardiac surgery.
Ultimately, a pulmonary shunt, either through an artificial prosthetic, diversion by a Blalock-Taussig shunt, or central shunt must be performed. These latter procedures involve surgical intervention with inherent anesthesia and perioperative risks.
Therefore, it is desirable to evaluate perfected cardiac catheterization techniques to establish a systematic pulmonary artery communication. What has been attempted in the prior art is the placement of stents in the ductal arteriosus using conventional angioplasty catheters. See Coe, "Redilating Ductal Stents in Newborn Lambs," 2163 Abstracts from 64th Scientific Sessions II-545; Moore et al., "Use of an Intravascular Endoprosthesis (stent) to Establish and Maintain Short Term Patency of the Ductus Arteriosus in Newborn Lambs," JACC Vol. 17 No. 2 (1991); and Coe et al., "A Novel Method to Maintain Ductal Patency," JACC Vol. 17 No. 2 (1991).
In Coe's study in newborn lambs, catheters were introduced via a cutdown in the neck under general anesthesia. A stainless steel stent which was securely mounted on a coronary angioplasty catheter, was introduced into the external jugular vein and manipulated through the right heart under fluoroscopy to place the balloon carefully in the ductus arteriosus. The position of the ductus was previously determined angiographically. The balloon on the catheter was inflated to release the stent in the ductus arteriosus without embolisation.
The difficulty with this technique, however, is proper placement in the ductus arteriosus which is a very small and short tubular shunt in the infant's cardiac circulation system near the heart. The stent, although made of stainless steel, is very thin and can be seen only with difficulty under fluoroscopy, which is the only means for determining when and if the stent is properly placed. Improper placement within the ductus arteriosus could result either in failure of the stent to maintain the ductus arteriosus open and/or possible creation of clotting site with resulting complications from embolism.
The inclusion of radiographic markers, either on the stent or the delivery catheter, helps determination of the location of the stent, but provides no assistance in determining whether or not the stent is properly placed with ductus.
Therefore, what is needed is an apparatus and method which can be used in combination with conventional fluoroscopy to allow accurate and secure placement of a stent in the ductus arteriosus of an infant.